1. Field of the Invention
This invention relates to a sheet conveying device applied in an image recording apparatus such as a printer, an electronic typewriter, an ink jet printer or a word processor. More particularly, the present invention relates to a sheet conveying device which is capable of automatic feeding and manual feeding of a sheet and in which, during automatic feeding, the detent force is not applied to a driving element as a load.
The word "sheet" used herein includes, for example, typewriting paper on which recording such as printing is effected, recording paper for word processors, and a transparent plastic sheet for an overhead projector (OHP).
2. Description of the Related Art
The conventional sheet feeding mechanism provided with a detent mechanism has adopted a structure in which a gear as the detent mechanism is provided on a paper feeding portion, for example, a platen shaft, and in which a ratchet is brought into mesh engagement with this gear.
When sheet feeding is to be effected by a motor or the like, the platen shaft is rotated by the motor. Thus, the gear integral with the shaft is rotated and the ratchet is disengaged from the gear over the teeth of the gear, so that sheet feeding is accomplished.
The provision of a detent mechanism is for the purpose of effecting click rotation relative to the rotation of the platen and also for causing the platen to be rotated by a predetermined pitch in accordance with the pitch determined by the pitch of the teeth of the gear of the detent mechanism, thereby enabling accurate sheet feeding to be accomplished.
In such a structure, the ratchet engages the gear integral with the platen shaft and acts in conjunction with the loads of the platen and the recording sheet urged against the platen by a pinch roller, so that the positioning of the platen may be effected. Therefore, the force needed to cause the ratchet to engage with the gear must increase as the rigidity of the recording sheet is increased.
Also, when sheet feeding is by motor, a load to feed the recording sheet and to cause the ratchet to be disengaged from the gear, i.e. over the teeth of the gear, is required, and this calls for a more expensive motor of increased output.
Furthermore, there has been a problem that the sheet feeding speed is reduced in order to increase the reduction gear ratio of a set of gears for transmitting the drive force.
As a means for solving such disadvantages, there has been proposed a structure in which a solenoid for moving the ratchet is provided, with so that when sheet feeding is to be effected by the motor, the ratchet is pulled apart from the gear. However, as previously mentioned, the force for causing the ratchet to engage with the gear is great, thus the solenoid for pulling the ratchet apart from the gear has also requires a great output. This has led to a problem that the device becomes bulky and costly.